In the field of anaesthetics, the trend is towards increasingly refined, and thus expensive, anaesthetics. It has consequently become desirable to reduce the consumption thereof.
One way of reducing the consumption of anaesthetics is to use an anaesthetic system with total or partial re-inhalation. In total re-inhalation, no gas escapes from the system, and the patient is breathing essentially the same gas mixture all the time. The carbon dioxide exhaled by the patient is eliminated in a CO.sub.2 absorber, and oxygen and the anaesthetic are supplied to maintain the concentrations thereof at the desired level. In partial re-inhalation, only part of the exhaled gas mixture is inhaled again, and a certain amount of fresh gas mixture is instead supplied. The gas surplus is discharged during exhalation. If the consumption of anaesthetics is to be minimized, a system with total re-inhalation is the best option.
Controlling the concentration of anaesthetics in anaesthetic systems with re-inhalation is today often performed manually. The anaesthetist sometimes may consult a gas-concentration meter mounted in the anaesthetic system and serving to measure the concentration of the anaesthetic.
It is also known to control the concentration of the anaesthetic automatically by means of a computer.
WO 88/06904 discloses an apparatus for controlling the concentration of a component in a gas mixture given to the patient. The apparatus includes a patient circuit which consists of a circle system for re-inhalation, which has an inhalation branch and an exhalation branch. A gas-concentration meter for measuring the concentration of the component in the gas mixture is provided in the inhalation branch close to the patient and is connected to a computer. A desired-value transducer for setting the desired concentration of the gas component is also connected to the computer. With the aid of signals from the gas-concentration meter and the desired-value transducer, the computer controls a gas-dosing unit which is provided further away from the patient in the inhalation branch, to give the desired concentration of the gas component. Thus, this is a feedback control.
Both in manual and computer-controlled operation it is, however, difficult to rapidly and highly accurately adjust the concentration of the anaesthetic to the desired level.
This is, among other things, due to the fact that there may be a considerable discrepancy between the concentration of anaesthetic in the flow supplied to the breathing system and the concentration of anaesthetic in the gas mixture inhaled by the patient.